This submission is migrated from previous version of oj.uz, which used different machine for grading. This submission may have different result if resubmitted.
#include <bits/stdc++.h>
using namespace std;
#define NAME "differ"
const int N = 1e6 + 5;
vector<int>pos[30];
int n,ans;
char s[N];
void calc(int a,int b) {
    vector<int>vec;
    int i = 0,j = 0;
    while (i < (int)pos[a].size() && j < (int)pos[b].size()) {
        if (pos[a][i] < pos[b][j]) {
            vec.push_back(1);
            i++;
        }
        else {
            vec.push_back(-1);
            j++;
        }
    }
    while (i < (int)pos[a].size()) {
        vec.push_back(1);
        i++;
    }
    while (j < (int)pos[b].size()) {
        vec.push_back(-1);
        j++;
    }
    int sum = 0;
    for (auto x: vec) {
        sum = max(sum + x,x);
        ans = max(ans,sum);
    }
}
signed main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0); cout.tie(0);
    cin >> n;
    for (int i = 1; i <= n; i++) {
        cin >> s[i];
        pos[s[i] - 'a'].push_back(i);
    }
    for (int i = 0; i < 26; i++)
        for (int j = 0; j < 26; j++) 
            if (i != j && pos[i].size() && pos[j].size()) calc(i,j);
    cout << ans << "\n";
}
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